Early adaptations in somatosensory cortex after focal ischemic injury to motor cortex.

In response to a lesion, intact regions of cortex in both hemispheres undergo adaptive changes in network function. For example, changes in excitability and intracortical inhibition in primary motor cortex (M1) were reported after lesioning contralateral or ipsilateral brain regions. Close interactions exist between M1 and primary somatosensory cortex (S1) within one hemisphere. Therefore, we hypothesized that lasting modifications would occur in S1 excitability after lesioning ipsilateral M1. Imaging of intrinsic optical signals (IOS, at 570 nm) was used to investigate the evolution of the somatosensory cortical response evoked by contralateral median nerve stimulation during the first hour after a photothrombotic lesion to M1 (caudal motor cortex) of the rat (n=10). Control rats (n=6) received no lesion. Perfusion was monitored by Laser speckle imaging and the extent of the resulting lesion was determined histologically. Control animals did not show evidence for reduced perfusion, infarction, or changes in IOS. M1 infarction led to a significant increase in evoked response amplitude, duration, and area of activation, and a shortening of latencies. These parameters reached a plateau around 50 min after ischemia. These results indicate S1 hyperexcitability after M1 injury. Whether these adaptations contribute to functional deficits or play a role in recovery, remains to be determined.